Limiting gep merging to fix the performance problem described in

https://llvm.org/bugs/show_bug.cgi?id=23163.

Gep merging sometimes behaves like a reverse CSE/LICM optimization,
which has negative impact on performance. In this patch we restrict
gep merging to happen only when the indexes to be merged are both consts,
which ensures such merge is always beneficial.

The patch makes gep merging only happen in very restrictive cases.
It is possible that some analysis/optimization passes rely on the merged
geps to get better result, and we havn't notice them yet. We will be ready
to further improve it once we see the cases.

Differential Revision: http://reviews.llvm.org/D8911

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235455 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index 3b46156c1ab0f7eab39d10a5785777d09f06557c..f62941fbf64268262cb4fad0966a6ffa91dd3fb3 100644 (file)
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1467,6 +1467,11 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
         // normalized.
         if (SO1->getType() != GO1->getType())
           return nullptr;
+        // Only do the combine when GO1 and SO1 are both constants. Only in
+        // this case, we are sure the cost after the merge is never more than
+        // that before the merge.
+        if (!isa<Constant>(GO1) || !isa<Constant>(SO1))
+          return nullptr;
         Sum = Builder->CreateAdd(SO1, GO1, PtrOp->getName()+".sum");
       }
 

diff --git a/test/Transforms/InstCombine/descale-zero.ll b/test/Transforms/InstCombine/descale-zero.ll
index 4347be48e618eff8e90569bf3681cc4cedbe0722..e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 100644 (file)
--- a/test/Transforms/InstCombine/descale-zero.ll
+++ b/test/Transforms/InstCombine/descale-zero.ll
@@ -1,20 +0,0 @@
-; RUN: opt < %s -instcombine -S | FileCheck %s
-
-target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
-target triple = "x86_64-apple-macosx10.10.0"
-
-define internal i8* @descale_zero() {
-entry:
-; CHECK: load i8*, i8** inttoptr (i64 48 to i8**), align 16
-; CHECK-NEXT: ret i8*
-  %i16_ptr = load i16*, i16** inttoptr (i64 48 to i16**), align 16
-  %num = load i64, i64* inttoptr (i64 64 to i64*), align 64
-  %num_times_2 = shl i64 %num, 1
-  %num_times_2_plus_4 = add i64 %num_times_2, 4
-  %i8_ptr = bitcast i16* %i16_ptr to i8*
-  %i8_ptr_num_times_2_plus_4 = getelementptr i8, i8* %i8_ptr, i64 %num_times_2_plus_4
-  %num_times_neg2 = mul i64 %num, -2
-  %num_times_neg2_minus_4 = add i64 %num_times_neg2, -4
-  %addr = getelementptr i8, i8* %i8_ptr_num_times_2_plus_4, i64 %num_times_neg2_minus_4
-  ret i8* %addr
-}

diff --git a/test/Transforms/InstCombine/getelementptr.ll b/test/Transforms/InstCombine/getelementptr.ll
index 9bb1b12a6972405a2bbdb802be7430f5c59e37d2..276ada91f3c63223f5240106306246cbe526b6c0 100644 (file)
--- a/test/Transforms/InstCombine/getelementptr.ll
+++ b/test/Transforms/InstCombine/getelementptr.ll
@@ -104,8 +104,8 @@ define i32* @test7(i32* %I, i64 %C, i64 %D) {
         %B = getelementptr i32, i32* %A, i64 %D
         ret i32* %B
 ; CHECK-LABEL: @test7(
-; CHECK: %A.sum = add i64 %C, %D
-; CHECK: getelementptr i32, i32* %I, i64 %A.sum
+; CHECK: %A = getelementptr i32, i32* %I, i64 %C
+; CHECK: %B = getelementptr i32, i32* %A, i64 %D
 }
 
 define i8* @test8([10 x i32]* %X) {